(1) Field of the Invention
The present invention relates to a method of fabricating semiconductor devices, and more particularly to a method of fabricating a semiconductor device involving a step of forming plugs in contact holes.
(2) Description of the Related Art
A prior art method of forming a contact plug using a polysilicon film (hereinafter referred to as the first prior art) will be described with reference to FIGS. 1A to 1D. First, as shown in FIG. 1A, a BPSG (boronphosphosilicate glass) film 102 is grown on a silicon substrate 101. Then, photosensitive etching resist (hereinafter referred to as a photoresist) 103 is coated, and its portion corresponding to a contact hole formation area is removed by using a photolithographic technique. Then, a contact hole 104 is formed using a dry etching process, and thereafter the photoresist 103 is removed.
Subsequently, as shown in FIG. 1B, a polysilicon layer 105 is grown such that it fills the contact hole 104 and has a substantially flat surface. Thereafter, this semiconductor wafter (SDW) is set on a lower electrode 703 of a dry etching apparatus which has a pair of upper and lower parallel electrodes 702 and 703 arranged in a chamber 701 as shown in FIG. 7. The lower electrode 703 is connected to a radio frequency (RF) power source 705 via a matching box 704. In this etching apparatus, a first stage of etching is carried out under the conditions of SF.sub.6 set to 100 sccm, a pressure set to 360 mTorr and an RF power density set to 2.7 W/cm.sup.2 until the residual film of the polysilicon film 105 on the BPSG film 102 becomes 0.1.about.0.2 .mu.m. In a subsequent second stage of etching, the polysilicon film 105 is etched under the conditions of CF.sub.4 set to 50 sccm, the pressure set to 360 mTorr and the RF power density set to 2.7 W/cm.sup.2 until the BPSG film 102 is exposed, as shown in FIG. 1C. Thus, a contact plug 106 is formed.
Finally, as shown in FIG. 1D, a silicide film 107 is formed using a spattering device or the like, and metal interconnects are formed.
In the above first prior art, great plug loss 108 is expected, as shown in FIG. 1C. Therefore, it is highly possible that the subsequent wiring formation results in breakage 109 of wiring as shown in FIG. 1D. The second prior art which has been intended to cope with this deficiency will be described with reference to FIGS. 2A to 2D.
First, as shown in FIG. 2A, a BPSG film 202 is grown on a silicon substrate 201, and then a photoresist 203 is coated and removed for a contact hole formation area using the photolithographic technique. Then, one-third to one-half of the thickness of the BPSG film 202 is wet etched, and the remaining BPSG film 202 is anisotropically dry etched, thus forming a contact hole 204. The resist 203 is then removed.
Then, as shown in FIG. 2B, a polysilicon film 205 is formed to fill the contact hole 204. Then, as shown in FIG. 2C, the polysilicon film 205 on the BPSG film 202 is dry etched, thus forming a contact plug 206. Finally, as shown in FIG. 2D, wiring is formed from a silicide film 207.
In this contact plug formation process, the contact hole 204 is formed by isotropic wet etching so that the contact hole 204 has a tapered opening portion. It is thus possible to form a wiring interconnect free from breakage irrespective of slight plug loss generation.
Where a contact plug is formed in the above first prior art, the amount of plug loss in the contact hole is, as shown in FIG. 3, 0.075 .mu.m with a contact hole diameter of 1.0 .mu.m, 0.5 .mu.m with a contact hole diameter of 0.8 .mu.m, 0.6 .mu.m with a contact hole diameter of 0.6 .mu.m and 0.85 .mu.m with a contact hole diameter of 0.4 .mu.m. This means that the prior art process of contact plug formation has a problem in that the range of contact hole diameter which is free from breakage 109 of wiring is 1.0 .mu.m or above and, if the contact hole diameter is less than 1.0 .mu.m the process cannot be applied because of the possibility of the breakage 109 of wiring due to the increase in plug loss.
The reason why the plug loss begins to occur as the contact hole diameter becomes less than 1.0 .mu.m is that, when the BPSG film 102 underlying the polysilicon film 105 is exposed and the polysilicon film 105 remains only in the contact hole, there is a sudden increase in the rate of etching of the polysilicon film 105. Experiments conducted by the inventor reveal that under the conditions of the second stage etching in the prior art (i.e., when the rate of etching of the BPSG film is one half or below the rate of etching of the polysilicon film), the rate of etching of the polysilicon film is high when the film is present only in the contact hole compared to the case when the film is entirely covering the BPSG film, as shown in FIG. 4. This trend is more pronounced with smaller contact hole diameters and becomes highly significant when the contact hole diameter (CHD) becomes less than 0.4 .mu.m. Within the accuracy of the currently adopted etching end detection technique, certain overetching is necessary in order to eliminate etching residue on the BPSG film. Therefore, great plug loss is prone particularly with contact plug of small size.
Further, in the first prior art process, as shown in FIG. 1B, the polysilicon film 105 is grown until it becomes flat over the entire semiconductor wafer surface before execution of the etching-back. However, with a contact hole diameter of 0.4 .mu.m or below, it is necessary, in order to obtain the flatness, to grow the polysilicon film 105 to a thickness twice the contact hole diameter if the diameter is 0.4 .mu.m and three times or above if the diameter is 0.3 .mu.m to an aspect ratio of 3, for instance. Therefore, it is necessary to etch a very thick film to form a small size contact plug. This means that, in the formation of a small diameter contact plug, the reproducibility of etching is deteriorated, thus making the etching end detection far more difficult.
In the second prior art noted above, the above problem in the first prior art is solved to a certain extent. However, in the second prior art process, wet etching is adopted to form the contact hole 204. Therefore, if the photoresist 203 and the BPSG film 202 are in an imperfect adhesive state, that is, if they are not closely in contact with each other, the etching proceeds through the semiconductor wafer to a greater extent in the horizontal direction than in the vertical direction, as shown in FIG. 5, and therefore it has been difficult to form an isotropic contact hole as shown in FIG. 2A with good reproducibility. In addition, since the wet etching is used, there is a limitation imposed on the miniaturization of the semiconductor device.